The invention relates to zeolites of X type, to their preparation and to their use in the separation of gas mixtures and more particularly to zeolites of type exchanged with lithium and with trivalent and/or divalent ions which are selective for nitrogen and which have an improved thermal stability and an improved crystallinity, to their preparation and to their use in the separation of nitrogen from less strongly adsorbed gases.
The separation of nitrogen from other gases, such as, for example, oxygen, argon and hydrogen, is of considerable industrial importance. When the separation is carried out on a large scale, fractional distillation is often used. However, distillation is very expensive because of the high initial costs for the plant and of the considerable energy demand which it involves. Other separation methods have recently been studied in efforts to reduce the overall cost of these separations.
An alternative to distillation which has thus been used to separate nitrogen from other gases is adsorption. For example, a sodium zeolite X, disclosed in U.S. Pat. No. 2,882,244, has been used with a degree of success for the separation by adsorption of nitrogen from oxygen. One disadvantage of the use of sodium zeolite X for the separation of nitrogen from oxygen is that it only has a low separating efficiency in the separation of nitrogen.
According to U.S. Pat. No. 3,140,933, an improvement in the adsorption of nitrogen is obtained when some of the base ions are replaced by lithium ions. This patent states that the zeolite of X type having base ions replaced by lithium ions can be efficiently used to separate nitrogen from oxygen at temperatures ranging up to 30xc2x0 C. Because the exchange of ions is not total and because the zeolites X have been synthesised in a sodium medium, the adsorbent used is a mixed sodium/lithium zeolite.
U.S. Pat. No. 4,859,217 discloses that very good separation of nitrogen from oxygen can be obtained by absorption at temperatures of 15 to 70xc2x0 C. using a zeolite of X type which has more than 88% of its ions in the form of lithium ions, in particular when a zeolite is used with a silicon/aluminium atomic ratio of between 1 and 1.25.
Unfortunately, the zeolite of X type highly exchanged with lithium has a very strong affinity for water and the presence of adsorbed water, even small amounts, seriously reduces the adsorption capacity of the zeolite. Consequently, in order to ensure optimum performance as regards adsorption, it is necessary to activate the zeolite by heating it to temperatures ranging up to 600 to 700xc2x0 C. in order to drive off as much adsorbed water as possible. Because the zeolites of X type exchanged with lithium are not stable at temperatures greater than approximately 740xc2x0 C., the activation of these adsorbents must be carefully controlled in order to prevent them from being damaged. Another disadvantage of zeolites of X type highly exchanged with lithium stems from the fact that they have a high production cost due to the price of the lithium compounds needed in their manufacture.
A need thus exists for adsorbents which have good thermal stability, good crystallinity and adsorbent properties for nitrogen at least equal to those of zeolites highly exchanged with lithium but which can be produced at more reasonable costs.
U.S. Pat. No. 5,179,979 maintains that lithium/alkaline earth metal zeolites of X type having lithium/alkaline earth metal molar ratios of the order of 95/5 to 50/50 approximately have a higher thermal stability than that of the corresponding zeolites with pure lithium and good adsorption selectivities and capacities.
U.S. Pat. No. 5,152,813 discloses the adsorption of nitrogen from gas mixtures which uses crystalline zeolites X having an Si/Al zeolite ratioxe2x89xa61.5 in which the exchangeable sites are occupied by at least 2 ions: between 5 and 95% of lithium ion and between 5 and 95% of a second ion chosen from calcium, strontium and mixtures of these, the total (lithium and second exchangeable ion) being at least 60%.
U.S. Pat. No. 5,464,467 or EP 667 183 provide a zeolite of X type, the cations of which comprise, referred to as equivalents, from approximately 50 to approximately 95% of lithium, from approximately 4 to approximately 50% of trivalent ions chosen from aluminium, scandium, gallium, iron(III), chromium(III), indium, yttrium, lanthanides alone, mixtures of two lanthanides or more, and mixtures of these, and from 0 to approximately 15% of residual ions chosen from sodium, potassium, ammonium, hydronium, calcium, strontium, magnesium, barium, zinc, copper(II) and mixtures of these, which is prepared by exchange of the exchangeable cations of the zeolite, preagglomerated with a binder, first with lithium and then with the trivalent cation or cations.
U.S. Pat. No. 5,932,509 provides for the preparation of these same zeolites according to a process which consists first in exchanging the exchangeable cations of the powdered zeolite X with trivalent cations, in then agglomerating with a binder and finally in carrying out the lithium exchange on the agglomerated zeolite.
The Applicants have found that the zeolites prepared according to the teaching of U.S. Pat. Nos. 5,179,979, 5,152,813, 5,464,467 or 5,932,509, although exhibiting good thermal stability and a good nitrogen adsorption capacity, have an insufficient crystallinity and exhibit a degree of heterogeneity in the distribution of the tri- and/or divalent cations.
The present invention provides zeolites of X type having an Si/Al atomic ratio of less than 1.5 and preferably of between 0.9 and 1.1, the exchangeable cations of which comprise, referred to as equivalents,
from approximately 50 to approximately 95% of lithium ions,
from approximately 4 to approximately 50% of trivalent ions chosen from aluminium, scandium, gallium, iron(III), chromium(III), indium, yttrium, lanthanides or rare earth metals, alone or as a mixture, and/or of divalent ions chosen from calcium, strontium, zinc, copper, chromium(II), iron(II), manganese, nickel or cobalt, alone or as a mixture,
0 to approximately 15% of residual ions chosen from sodium, potassium, ammonium or hydronium, alone or as a mixture,
Which are capable of being obtained according to a process which comprises the following stages:
a) suspension of the zeolite in water, then
b) exchange of the exchangeable cations of the suspended zeolite with one or more di- and/or trivalent ions by simultaneous and/or successive contact(s) in a rapid mixer of the said suspension with one or more solutions comprising compounds of the di- and/or trivalent ions,
c) exchange of the exchangeable cations of the zeolite resulting from stage b) with lithium,
which stages will be explained in detail below.
The zeolites of the present invention can be provided in various forms and the exact form which they assume can determine their usefulness in the industrial adsorption processes. When the zeolites of the present invention are used in industrial adsorbers, it may be preferred to agglomerate (for example, to convert into granules) the zeolite in order not to risk compacting the pulverulent zeolite in an adsorption column of industrial size, thus blocking or at the very least greatly reducing the flow through the column. These techniques generally involved mixing the zeolite with a binder, which is usually a clay, converting the mixture to an agglomerate, for example by extrusion or bead formation, and heating the zeolite/binder mixture formed to a temperature of 600-700xc2x0 C. approximately in order to convert the xe2x80x9cgreenxe2x80x9d agglomerate to an agglomerate which is resistant to crushing. The binders used to agglomerate the zeolites can include clays, silicas, aluminas, metal oxides and their mixtures.
It is possible to prepare agglomerates comprising less than 10%, indeed even less than 5%, by weight of residual binder. A process for producing these agglomerates with a low level of binder consists in converting the binder of the agglomerates described above to the zeolite phase. For this, the starting point is the agglomeration of a zeolite LSX powder with a binder which can be converted to zeolite (for example kaolin or metakaolin), then conversion to zeolite is carried out by alkaline maceration, for example according to the process disclosed in EP 932 581, and then the granule which has been converted to zeolite is exchanged with sodium. It is thus possible to easily obtain according to the invention outstandingly effective agglomerates assaying at least 90%, indeed even at least 95%, of zeolite LSX.
In addition, the zeolites can be agglomerated with materials such as silica/alumina, silica/magnesia, silica/zirconia, silica/thoria, silica/beryllium oxide and silica/titanium dioxide, as well as with ternary compositions, such as silica/alumina/thoria, silica/alumina/zirconia and clays present as binders. The relative proportions of the materials and of the zeolites mentioned above can vary widely. When the zeolite has been converted to agglomerates before use, these agglomerates advantageously have a diameter of approximately 0.5 to approximately 5 mm.
The agglomeration binder generally represents from 5 to 30 parts by weight per 100 parts of agglomerate.
A second subject-matter of the invention relates to a process for the preparation of the zeolites defined above.
The zeolites of the invention are generally prepared from a base zeolite of X type which usually originally has sodium and/or potassium ions as charge-compensating cations, that is to say ions which compensate for the negative charge of the aluminosilicate lattice.
The exchangeable cations of the starting zeolite are exchanged with a solution of the compounds of the trivalent ions and/or of the divalent ions (stage b)) by simultaneously pumping the suspension of zeolite to be exchanged, suspended beforehand in water (stage a)), and the solution of the compounds by forcing them to pass through a rapid mixer capable of providing homogeneous mixing of the suspension and of the solution after a short time of contact between the suspension and the solution (a few minutes), which mixer is preferably a static mixer capable of providing homogeneous mixing after a very short contact time (a few seconds), all arrangements being made in order for the flow rates to be adjusted so as to retain a weight of suspension/weight of solution ratio which is virtually constant. Recourse to exchange in a static mixture has no effect with regard to the degree of exchange, which remains the quantitative degree achieved by conventional exchange. A better random distribution of the di- and/or trivalent ions within the zeolite structure is obtained, which is reflected by a final level for the nitrogen adsorption capacity which is significantly improved, which is entirely unexpected.
It is preferable, although this is not absolutely essential, to use aqueous solutions of the exchange ions. Any water-soluble compound of the exchanging ions can be used. The preferred water-soluble compounds of the ions are the salts and in particular the chlorides, the sulfates and the nitrates. The particularly preferred salts are the chlorides, because of their high solubilities and their ready availability.
When it is desired to prepare a zeolite according to the invention, a portion of the cationic sites of which are occupied by several types of divalent and/or trivalent ions defined above, it is possible either to simultaneously exchange all the cations, by contact with a solution comprising all these cations, or to carry out a successive exchange of each cation, or to use a solution intermediate between the 2 above solutions.
A preferred alternative for the invention consists in exchanging the exchangeable cation or cations with tri- and/or divalent ions and with monovalent ions, preferably the sodium ion.
Another preferred alternative form consists in stabilizing, with sodium hydroxide, the zeolite immediately after the exchange with tri- and/or divalent ions or else after the exchange with tri- and/or divalent ions and monovalent ions.
When the zeolites according to the invention are provided in agglomerated form, a particularly preferred form, the agglomerated stage is carried out after the exchange of a portion of the exchangeable cations of the zeolite with tri- and/or divalent cations, optionally in the presence of a monovalent cation.
It proves to be particularly advantageous to convert all the exchangeable cations of the starting zeolite to a single monovalent cationic species, preferably the sodium or ammonium ion form, prior to the exchange with the di- and/or trivalent cations as explained in detail above. For this, the zeolite is brought into contact with a solution comprising monovalent ions, such as sodium or ammonium, for example an aqueous NaCl or NH4Cl solution.
Once exchanged with di- and/or trivalent ions, the zeolite can also be advantageously brought into contact, before or after the optional agglomeration stage, with a solution comprising sodium or ammonium ions, for example an aqueous NaCl or NH4Cl solution.
The following stage of the process according to the invention (stage c)) consists in exchanging a portion of the exchangeable cations with lithium ions by contact with a lithium compound solution, preferably an aqueous solution of lithium salt, such as LiCl. In a way known to a person skilled in the art, the zeolite, after each ion exchange stage, is washed with water and then dried at a temperature generally of between 40 and 200xc2x0 C.
Preferably, at atmospheric pressure, the temperature of the solution of lithium compounds is between 80 and 115xc2x0 C. and in particular between 95 and 115xc2x0 C. Higher temperatures can be used, with pressurization of the system.
The concentration of lithium in the solution, limited by the solubility of the salts, is chosen to be as high as possible in order to reduce the cost related to reprocessing.
A particularly advantageous alternative form consists in carrying out the operation countercurrent-wise and continuously according to the procedure of EP 863 109 explained in detail below:
The zeolite is distributed in at least 2, preferably at least 3, receptacles in the stationary bed form which are arranged in series in an interchangeable manner, the solution of lithium compounds is conveyed through the said receptacles arranged in series and the sequence of the receptacles arranged in series, known as a xe2x80x9ccarouselxe2x80x9d, is modified cyclically at given time intervals, the inlet of the fresh solution being moved on each occasion from the 1st receptacle, in which is found the zeolite which has been exchanged with lithium to the greatest extent, to the following receptacle in the series; when the desired degree of exchange of lithium is reached for the zeolite in the 1st receptacle, the latter is taken out of the series of receptacles of the carousel and the zeolite which is present therein is freed from the solution of lithium compounds by washing, then discharged and optionally replaced by a fresh charge of zeolite to be exchanged.
The cycle time of a time interval typically amounts to at least 15 minutes and preferably to at least 1 hour.
According to a third subject-matter of the invention, the zeolites according to the invention are used as adsorbent of the nitrogen present in a gas mixture and in particular air, thus making it possible to separate the nitrogen from the other gases present in the gas mixture. The separation is carried out by passing the gas mixture into at least one adsorption region comprising the adsorbent according to the invention.
In a preferred embodiment, the adsorption process is cyclic and comprises the adsorption stage described above and a stage of desorption of the nitrogen from the adsorption region(s).
The preferred cyclic processes encompass adsorption by pressure variation (PSA for Pressure Swing Adsorption or VSA for Vacuum Swing Adsorption (specific case of desorption under vacuum)), by temperature variation (TSA for Temperature Swing Adsorption) and combinations of these (PTSA).
According to some preferred embodiments, the adsorption stage is carried out at a temperature of between approximately xe2x88x9220 and approximately 50xc2x0 C. and at an absolute pressure of between approximately 0.08 and 1 MPa.
In a highly preferred embodiment of the invention, the adsorption process is used to separate the nitrogen from a gas mixture comprising nitrogen and one or more other gases chosen from oxygen, argon, helium, neon and hydrogen.
In other preferred embodiments of the invention, the stage of regeneration of the adsorption zone is carried out by vacuum means (suction), by purging the adsorption zone with one or more inert gas(es) and/or with a part of the gas mixture coming out the adsorption zone, by temperature variation or by combination of these regenerations by suction, by purging and/or temperature variation; in general, the repressurization of the bed is at least partially carried out by using the nonadsorbed gas from the adsorption system.
The temperature at which the adsorption stage of the adsorption process is carried out depends on a number of factors, such as the specific gases to be separated, the specific adsorbent used and the pressure at which the adsorption is carried out. In general, the adsorption stage is carried out at a temperature of at least approximately xe2x88x9220xc2x0 C. and advantageously at a temperature of at least approximately 15xc2x0 C. The adsorption is generally carried out at temperatures which are not greater than approximately 70xc2x0 C. and preferably at temperatures which are not greater than 50xc2x0 C. and more preferably still at temperatures which do not exceed approximately 35xc2x0 C.
The adsorption stage of the adsorption process according to the invention can be carried out at any of the usual and well known pressures used in adsorption processes of TSA, PSA or VSA type. Typically, the minimum absolute pressure at which the adsorption stage is carried out is generally at least equal to atmospheric pressure. The maximum absolute pressure at which the adsorption stage is carried out generally does not exceed approximately 1.5 MPa and preferably approximately 1 MPa and advantageously approximately 0.4 MPa.
When the adsorption process is adsorption by pressure variation, the adsorbent is generally regenerated at a pressure of between 10 and 500 kPa, preferably from 20 to 200 kPa approximately, lower than the adsorption pressure and, when it is adsorption by temperature variation, it is generally regenerated at a temperature of between 0 and 300xc2x0 C. approximately, greater than the adsorption temperature.
When the adsorption process is of TSA type, the temperature of the bed(s) is thus increased during the regeneration stage with respect to the adsorption temperature. The regeneration temperature can be any temperature below that at which the adsorbent begins to decompose. Generally, the temperature of the adsorbent is usually increased to a value of between approximately 0 and approximately 300xc2x0 C. and preferably between approximately 25 and 250xc2x0 C. and advantageously between approximately 50 and 180xc2x0 C. The regeneration procedure can be a combination of PSA or VSA and of TSA, in which case both the temperature and the pressure used during the regeneration will vary within the ranges indicated above.